UVC devices with different specifications were included in this study to represent the variety of UVC devices available on the market. SARS-CoV-2 inactivation performance was examined and categorized by device application to provide ease of use in practice.
Different applications of UVC devices possess certain characteristics that make them useful for different purposes. Specifically, handheld UVGI surface disinfection equipment is characterized as a small portable UVC device providing a minimum sufficient level of UVC irradiance for SARS-CoV-2 inactivation. Therefore, this application is appropriate for disinfecting small surface areas, such as cell phones, keyboards, and door handles. The effective range is a short distance ranging from 1 to 10 cm. A practical misleading point for this application is the overestimation of the application range, as evidently seen in the M1 device. A longer duration of UVC irradiance could not compensate for a longer effective range.
UVGI disinfection chambers are another common UVC application suitable for surface disinfection. Importantly, the values of UVC irradiance at the top and bottom of the chamber were different. The UVC dosage at the bottom of the chamber in 3 devices (M3, M4, and L2) was more than the dosage required for SARS-CoV-2 inactivation; therefore, the recommended duration could be shortened. However, the design of the chambers can prevent potential UVC side effects in humans; thus, the authors the support manufacturers’ usage instructions to gain the benefit of inactivation of bacteria and other viruses without increasing the risk of side effects(Hijnen et al. 2006).
Comparing the 3 studied applications, the UVC dosages of the movable UVGI air and surface purifiers at the recommended settings were the highest, which can be explained by their usage purpose. These applications are effective not only for surface disinfection but also proper for air disinfection. Therefore, the expected range of the applications is longer than that of the other 2 applications. Unlike UVGI disinfection chambers, UVC irradiance from movable UVGI air and surface purifiers disperses, so potential side effects to humans should be taken into consideration. Accordingly, the authors suggest a shorter irradiance duration for movable UVGI air and surface purifiers in SARS-CoV-2 inactivation.
In addition to distance and duration, the direction of UVC irradiance is another crucial factor determining the disinfectant property of UVC. Boyce et al.(Boyce et al. 2016) conducted an experimental study to evaluate the impact of room location on UVC irradiance and UVC dosage. The results revealed that the orientation of the UVC sensor relative to the UVC device affected UVC irradiance. The UVC sensor pointed directly at UVC light yielded the highest UVC irradiance.
Another key concern in selecting UVC devices is the type of UVC light source. A low-pressure mercury lamp is a traditional UVC source mainly emitting at 254 mm(Anonymous 2020b). Although a low-pressure mercury lamp effectively generates UVC irradiance for disinfectant purposes, it has a major disadvantage that should not be overlooked. The main component (mercury) is known for its toxicity to humans and the environment(Bernhoft 2012, Clarkson &Magos 2006). As the accessibility of UVC devices expands, the public should be aware of mercury toxicity, and the safety of commercial mercury lamps must be ensured by manufacturers. LEDs are another UVC light source that were recently introduced to commercial markets. The use of LEDs is rising because they contain no mercury; thus, UVC LED devices are safe for human use and environmentally friendly. However, the limitations of LEDs are their small surface and directionality; hence, the use of UVC is limited for UVGI disinfection chambers and handheld UVGI surface disinfection equipment applications.
The authors are aware of the limitations of this study. First, a benchmark for the SARS-CoV-2-inactivating UVC dosage was used instead of examining SARS-CoV-2 inactivation with UVC. The second limitation is the generalizability of the results. The specifications of the studied UVC devices varied in the type of light source, application, and usage (distance and cycle duration), which could be inferred from the variety of UVC devices available in the commercial market. Consequently, the results from the present study will help guide the effectiveness of commercial household UVC devices for SARS-CoV-2 inactivation, but further adjustments are necessary depending on the specifications of the UVC device.